Re: 1991 735 Intermittent overheating (archive)

Your assessment that there is air in the system sounds right, especially as you say that the heater gurgles. Are you sure that all the air was bled from the system when the thermostat was replaced? When you bleed the system, the heater controls on the inside of the car have to be on maximum heat as the heater core is part of the coolant system.

Since the overheating occurs when the car is moving rather than idling, it sounds like coolant circulation is the problem. It may indeed be the water pump. Have you checked that there is nothing blocking the radiator?

Also check that your auxiliary fan is working properly (the fan behind the front grill). When the coolant temperature rises, this fan should switch on to provide extra cooling. It also runs continuously when the A/C is on. There is temperature switch on the upper right-hand side of the radiator that controls the speed of the fan (low or high). You can disconnect the switch and jumper the terminals together (with the ignition on) and the fan will run. There are three pins on the connector. Jumper the middle pin with each of the outer pins for low then high speed operation.

Let us know how things turn out.

Regards

Alex Lynch
1992 735i 194,000 miles
1972 2002 187,000 miles

About 6 times in the past 12 months I have found my temp. indicator going into the red. I have replaced my radiator, fan clutch, thermostat (two times). The car has 125,000 miles and I do not believe the water pump has ever been changed. On my last trip at about 250 miles into the trip on the highway without much stopping with the outside temp at about 20 degrees it occurred again. A local garage changed my thermostat (I later tested this thermostat in boiling water and it did open). They found that the bottom hose was not very hot. My return trip was fine, same outside temp. with much heavier traffic. Heater does smell like antifreeze when it is first turned on but smell goes away. Heater gurgles sometimes when it is turned on. BMW dealer last month could not find the probem on the pressure test. (I am not sure how hot they got the engine) I do have occasional minimal leaking of antifreeze on the floor but not so much that I am adding antifreeze. I suspect that air is getting in somewhere causing an air bubble and blocking the flow. Help.

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